Filter for continuous filtration

ABSTRACT

A filter for continuous filtration of a liquid includes conventionally a vessel having a center shaft on which is arranged at least one rotating filter element defining an inner space for liquid obtained at filtration. Further there is provided a washing device for washing of material deposited on the filter element, a connection between said inner space and outlet means from said vessel and a separator connected to said outlet and adapted for separating filtrate and gas. Means is provided outside said vessel for generating an overpressure in the gas space of the vessel and a lower pressure in the interior of the filter element at least corresponding to the evaporating temperature of the filtrated liquid for evaporation of the liquid in the separator. The separator is provided with a cooling device for cooling the separated gas and an outlet for condensate obtained at cooling.

FIELD OF THE INVENTION

The present invention concerns a filter for continuous filtration of aliquid by means of at least one filter element particularly but notexclusively adapted to be used as a filter in a causticizing process.

Similarly to prior art filters, a filter according to the presentinvention includes a vessel containing the liquid to be filtered and gasin a space located above the liquid level, at least one rotatable filterelement provided in the vessel and defining an inner space for filtratedliquid obtained by the filtration, a device for washing materialseparated from the liquid and deposited on the filter element,communication between said inner space of the filter element and anoutlet from said vessel, and a separator connected to said outlet forseparating the filtrated liquid and gas entrained therewith.

SUMMARY OF THE INVENTION

As distinguished from prior art filters a filter, according to thepresent invention is characterized in that for to provide the pressuredifferential necessary for filtration between the gas space of thevessel and the inner space of the filter element, means are providedoutside the vessel for generating an overpressure in the gas space ofthe vessel and a lower pressure in the inner space of the filter elementat least corresponding to the evaporation temperature of the filtratedliquid in order to keep the filtrated liquid in the state of evaporationin the separator, and in that the separator is provided with a devicefor cooling of the separated gas and with outlet means for dischargingcondensate obtained upon cooling.

According to the present invention, by providing said means forpressurizing and said separator with said cooling device and said outletmeans for condensate a number of advantages are obtained. Thus, a higherconcentration of filtrated liquid (filtrate) is produced because asubstantial part of the washing liquid (water) supplied for theabove-mentioned washing, which would otherwise dilute the filtratedliquid, is withdrawn by the condensation. Further, an improvedseparation of condensate and gas vapor is obtained, so that a smalleramount of gas has to be added to the filter system, which in turnreduces the risk of oxidation in the system of the medium passingthrough the filter system. In certain cases, condensate can be recycledto the washing device in which case it would not be necessary to supplywashing liquid and this may also result in a simplified structure.Further, a greater capacity is produced because in a higherrevolutionary speed of the filter element is possible.

In summary, the principal advantages of the present invention areoptimum expulsion of gas from the filtrated liquid and also an optimumexpelling of washing liquid.

One embodiment of the present invention also comprises a conduit betweenthe gas space of the separator and the gas space of the vessel forrecirculating gas to the gas space of the vessel, said gas conduitcomprising said means for generating said overpressure and said lowerpressure. This results in a simple embodiment for obtaining saidoverpressure and said lower pressure, and also a simple provision formutal adaption, especially when in a development of this embodiment saidpressure generating means is divided into at least two pressuregenerating means separated along the gas conduit, at least said meansprovided closest to the separator generating said lower pressure in saidspace within said filter element and a subsequent means increasing thepressure for obtaining the overpressure existing in the gas space of thevessel. In this last mentioned embodiment, prevailing atmosphericpressure may be preferably be at a point between said pressuregenerating means for obtaining said lower pressure and saidoverpressure, respectively. The higher concentration of filtrated liquidis also caused by a preferred embodiment wherein the filter elementcomprises at least one disc element mounted on a substantiallyhorizontal shaft, and wherein said washing device is arranged forwashing only of a zone the disc element located closest to the shaftsubstantially concentric relative thereto, and wherein said lowerpressure at least substantially solely prevails over said washing zone.

In a further preferred embodiment of the present invention, the washingdevice is divided into at least two separate washing means for washingin at least two steps, whereby occurs an advantageous stirring of thematerial deposited on the outside of the filter element at the washingin the second step involving a better filtration than in prior artstructures having only one washing means.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other advantages of the present invention willbe apparent from the following description of embodiments of filtersaccording to the present invention, reference being made to the annexeddrawings, wherein FIGS. 1-4 schematically show front elevations of fourembodiments of the filters according to the invention.

DETAILED DESCRIPTION

The filters shown in the drawings are so-called disc filters, whichconventionally include like a substantially cylindrical vessel 1, acenter shaft 2 rotatably journalled in the vessel and a number of filterdiscs (not shown) arranged on the center shaft spaced from from eachother, as shown e.g. in U.S. Pat. No. 4,695,381. Each filter disc iscomposed of a number of disc sectors (not shown), for as shown in thesaid U.S. Pat. No. 4,695,381, said filter sectors in a conventionalmanner being composed of surrounding filter cloth and internal channelsand being connected to the center shaft for discharging therethroughfiltrate and gas resulting from the filtration. The center shaft may beformed with a number of axial channels, one channel for each filtersector, as described and shown in the above-mentioned Patent, or maycomprise a single channel for filtrate and gas from all filter sectorsof the pressure vessel, as shown in published PCT ApplicationPCT/SE90/00413 (WO/90/10490). Further as also shown in the latter, thefilters may include a removing device or scraper for removal of filtercake deposited on the filter cloth during filtration and a device forremoving filter cake material from the cloth located inside scraped offmaterial, so-called precoat layer. As shown, e.g., in FIGS. 1 and 3, thefilters further comprise a device 3 for receiving the scraped-off filtercake material and discharging it through the conduit 4 by means of apump 5, and, as shown in FIGS. 3 and 4, a device 6 for washing of thefilter cloth. Further, the filter vessel has an inlet 7 for supplyingthe liquid to be filtrated and to keep the liquid at a certain level 8in the vessel.

As shown in FIGS. 1-3, filtrate and gas obtained upon filtration aredischarged through a conduit 9 to a separator 10. In the embodimentaccording to FIG. 4 intermediate walls are arranged in the center shaft2 is in order to obtain on the one hand a prefiltrate when the filterdiscs pass through the liquid, said prefiltrate being discharged to afiltrate tank 12 through a conduit 13, and on the other hand a clearfiltrate discharged to said filtrate tank through conduit 14, and, whenthe filter discs pass through the space above the liquid level filtrate,washing liquid and gas discharged through conduit 15 to the separator10. In the separator 10, filtrate and gas are separated, the filtratebeing discharged through conduit 16 by means of pump 17 and the gasbeing recirculated through conduit 18 to the vessel 1.

In the gas conduit 18 are arranged two blowers 19 and 20, blower 19being adapted to generate a lower pressure in the interior of the filterdiscs and the blower 20 being adapted to generate an overpressure in thegas space of vessel 1 above the liquid level 8. These blowers 19, 20constitute examples of means for obtaining the pressure differentialnecessary for filtration between the gas space of the vessel and theinterior of the filter discs by an overpressure in the gas space of thevessel and a lower pressure in the interior of the filter discs. Herebyis created a filter having advantages obtained only in a pressure filteras well as advantages obtained only in, for instance, a vacuum filter.Hereby is obtained, for instance, substantial conversion and cleaning ofgas circulating in the filter system. Further, in comparison to pressurefilters, a substantially cheaper filtration vessel can be utilized. Theinvention further permits the use of simpler and cheaper means forgenerating overpressure and underpressure, respectively, than in toprior art known pressure and vacuum filters, respectively. Also a fasterpressurizing of the filter is acheived.

As appears from FIGS. 1-3, a valve 21 is located between the blowers 19and 20, and after the blower 20 a valve 22 downstream of blower 20. Thevalve 21 is adapted to provide a pressure of substantially zero betweenthe blowers, and the valve 22 is adapted to prevent an excessivepressure in the gas space of vessel 1. In the embodiment of FIG. 4, aconduit 23 leads from a point between blowers 19 and 20 to the space ofthe filtrate tank 12 above its liquid level 24, said space being ventedto the surrounding atmosphere through conduit 25. Overpressure and lowerpressure can be varied in adapted to a given filtration situation. Thelower pressure is adapted to the temperature of the filtrate in theseparator 10 in order to produce an optimum boiling effect of theliquid. The overpressure is adapted to the lower pressure, such that apressure differential over the filter element necessary and suitable forfiltration is obtained. For instance, in the case of white liquorfiltration, the pressure differential may be 1.0 bar, the overpressurein the gas space of vessel 1 then may be between 0.1 and 0.9 bar and thelower pressure in the interior of the filter discs between 0.9 and 0.1bar.

As shown in the drawings figures, the separator 10 is provided with acooling device 26, in this embodiment a device cooled by cooling water,introduced through conduit 27 and discharged through conduit 28 providedwith valve 29. The condensate obtained by cooling is discharged throughconduit 30.

Another significant feature of a filter according to the presentinvention is shown in FIGS. 3 and 4, and This novelty comprises animproved washing device referred to in 6 divided into at least twoseparate washing means, e.g., spraying tubes, for washing in at leasttwo steps. The second washing step produces stirring of the materialdeposited on the outside of the filter discs, improves filtration overprior art devices having only one washing means. As also appears fromFIGS. 3 and 4, the location of the washing means 6, as well as thelocation of the device 3 for receiving the scraped-off filter cakematerial, depends on the rotational direction of the filter discs.Washing liquid for the washing means can either be supplied fromoutside, for instance as shown, only to the first washing means 6,through conduit 31 having a valve 32, or condensate from the separator10 may be supplied to a washing means, for instance as shown, throughconduit 33 to the second washing means 6, in this case a pump 34 beinginserted in condensate conduit 30 and a valve 35 being inserted inconduit 33.

While the invention has been described and illustrated in connectionwith disc filters, it may also be used with drum filters, i.e., a filterelement in the shape of a rotating drum having a surrounding filtercloth.

I claim:
 1. A filter for continuous filtration of a liquid underpressure, said filter including a vessel containing said liquid to acertain level and gas in a gas space located above said liquid, at leastone rotatable filter element provided in said vessel and defining aninner space for filtrated liquid obtained by filtration, a washingdevice for washing material separated from said liquid and deposited onsaid filter element, communication between said inner space of saidfilter element and outlet means from said vessel for discharging saidfiltrated liquid and gas entrained therewith, and a separator connectedto said outlet for separating said filtrated liquid from entrained gas,means being provided outside said vessel for generating overpressure insaid gas space of said vessel and a lower pressure in said inner spaceof said filter element at least corresponding to an evaporationtemperature of said filtrated liquid in order to maintain said filtratedliquid in a state of evaporation within said separator, said separatorbeing provided with a cooling device for cooling the separated gas andwith outlet means for discharging condensate obtained upon cooling.
 2. Afilter according to claim 1, comprising a gas conduit disposed between agas space of said separator and said gas space of said vessel forrecirculating gas to said gas space of said vessel, said means forgenerating said overpressure and said lower pressure being arranged insaid gas conduit.
 3. A filter according to claim 2, wherein saidpressure generating means comprises at least two pressure generatingmeans located in said gas conduit, at least one of said pressuregenerating means being arranged closest to said separator and generatingsaid lower pressure in said inner space of said filter element, and asubsequent one of said pressure generating means increasing the pressurefor obtaining said overpressure prevailing in said gas space of saidvessel.
 4. A filter according to claim 3, comprising pressure regulatingmeans disposed in said gas conduit and adapted to maintain atmosphericpressure in said gas conduit between said pressure generating means forobtaining said lower pressure and said overpressure, respectively.
 5. Afilter according to claim 1, wherein said filter element comprises atleast one disc filter mounted on a substantially horizontal shaft, saidwashing device being adapted for washing only one washing zone of saiddisc filter located closest to said shaft and substantially concentrictherewith, said lower pressure prevailing at least substantially oversaid washing zone.
 6. A filter according to claim 1, wherein saidwashing device comprises at least two separate washing means for washingin at least two steps.
 7. A filter according to claim 1, wherein atleast a part of the condensate obtained by cooling in said separator isrecirculated to said washing device.
 8. A filter according to claim 1,wherein said pressure generating means comprises blowers.